For a few years, thoughts have turned to the increase in traffic and the ensuing loading of air traffic controllers. In order to guarantee the safety and also the economic viability of air transport, it is envisaged, notably in the approach phase, that a time constraint be imposed on a particular waypoint: runway threshold, Initial Approach Fix (IAF), or rallying point for final approach, termed the ATC Merge Point.
This allows the air traffic control to guarantee a smoothed flow in the approach, and to manage a stable number of aeroplanes corresponding to the capabilities of the ground facilities and to the limit loading of an air traffic controller.
These time constraints can also serve in other operational contexts such as the management of the number of aeroplanes per sector.
Aboard the aircraft, the time constraint is in general inserted into a flight management computer termed the FMS (the acronym standing for Flight Management System). A flight management system consists of various functional hardware components which allow the crew to programme a flight using a navigation database. The system calculates a lateral and vertical trajectory making it possible to reach the destination of the flight plan. These calculations are based on the characteristics of the aeroplane and data provided by the crew and the environment of the system. The positioning and guidance functions collaborate to aid the aircraft to remain on this trajectory.
The pilot can programme the meeting of a time constraint, termed RTA for Required Time Arrival, at a point of the flight plan on the request of air traffic control for example. In this case the FMS performs an optimization of the trajectory by successive iterations so as to comply with the constraint.
To comply with an RTA, the FMS calculates predictions to determine the speed strategy. Once the strategy has been chosen, a re-calculation will take place if the prediction for the time of transit at the constrained point, termed the ETA for Estimated Time of Arrival, departs from a predetermined tolerance.
However the speed of an aircraft is confined within a speed envelope defined by two speed profiles: a maximum speed profile and a minimum speed profile. They depend mainly on the weight and the altitude of the aircraft. The maximum speed also depends on the ambient temperature. Other parameters can also come into play depending on the type of aircraft. FIG. 1 represents a typical evolution of the limit values of a flight envelope 11 for a given altitude and temperature as a function of the aircraft weight. The abscissa axis represents the weights decreasing towards the right, the ordinate axis the speeds. It is noted that the minimum speed VMIN increases with the weight of the aircraft, while its maximum speed VMAX decreases onwards of a certain threshold.
In the flight management systems according to the known art, the speeds are expressed in a speed unit called CAS, the acronym standing for Calibrated AirSpeed, or in MACH. Nevertheless, the meeting of a time constraint is dependent on the ground speed or GS. The ground speed is the horizontal component of the speed relative to the ground; it is determined by the sum of the air speed and of the wind. FIG. 2 represents the variation of the air speed as a function of altitude for a given speed expressed in terms of CAS or MACH. It may be noted that for a constant value of CAS, the air speed (and therefore the ground speed) increases with altitude. For a constant value of MACH, the ground speed decreases with altitude. Speed alterations are made rather in terms of CAS at low altitude and in terms of MACH at high altitude.
In the flight management systems according to the known art, the speed setpoints are limited to: a CAS/MACH pair for the aircraft climb phase, a few MACH speed values for its cruising phase and a CAS/MACH pair for the descent phase.
The setpoint CAS and MACH are dependent on an economic optimization criterion termed CI for Cost Index, weight, altitude, and temperature.
The Cost Index is in fact a criterion for optimizing between the time costs CT (“Cost of Time”) and the fuel costs CF (“Cost of Fuel”). The Cost Index is defined by CI=CT/CF. The value of this cost index for an aircraft and a given mission is determined according to criteria specific to each operator, and constrains notably the rules for determining the altitudes and speeds of the flight plan (vertical profile of the flight plan).
The maximum speeds (CAS or Mach) may be dependent on the weight and the altitude on certain aircraft, as is the case for FIG. 1. FIG. 2 presents a curve of minimum speed Vmin 201 and a curve of maximum speed Vmax 202 corresponding to a case of initial weight denoted GW0, and integrating the lightening of the weight of the aircraft. The minimum speeds (CAS or Mach) take account of the stall speeds with a margin. These minimum speeds are dependent notably on the weight, altitude and temperature.
The CAS and MACH setpoints, calculated with the schemes according to the known art, are limited by the envelope. Each of these limits is calculated for a single point of the envelope. For the climb phase or the descent phase, it may happen that the flight envelope at the top or at the bottom is more limiting than the flight envelope during the phase.
Several schemes according to the known art make it possible to control the 4D trajectory of the aircraft so as to make it comply with a time constraint. These schemes all perform a convergence in speed, in open loop: the 4D trajectory is reoptimized at regular intervals but is not regulated. These schemes are generally based on a variation of the Cost Index.
A flight management system making it possible to comply with a time constraint by varying a cost index “Cost Index” is known through U.S. Pat. No. 5,457,634. Such a system makes it possible notably to calculate an optimal cruising altitude so as to economize on fuel consumption. One of the drawbacks of such a system arises when a time constraint cannot be complied with. The system can then signal that the constraint is deficient though the latter could be complied with by adopting a flight speed closer to the limits of the flight envelope.
The invention is aimed at alleviating the problems cited previously by proposing a method for calculating a speed making it possible to comply with a time constraint RTA. The invention consists no longer in calculating a single CAS/MACH pair during climb/descent but in adapting the speed in a continuous manner to the bounds of the curves of minimum Vmin and maximum Vmax speeds when a time constraint may not be achieved by following a single CAS/MACH pair.